**Acknowledgements**

current no more than 2–3 times when the temperature lowers from 300 to 260–270 K, and further cooling loses its meaning; (iii) with thinning the semiconductor crystal, the ratio between the carrier drift length and crystal thickness increases that improves the efficiency of charge collection at relatively low bias voltage; (iv) in the CdZnTe and CdMnTe detectors under study, a rapid rise of the current with increasing voltage and temperature due to injection of charge carriers is observed, so these crystals are not suitable for fabrication of X/γ-ray detectors; (v) a significant increase in the energy resolution can be achieved by improving the quality of Cd(Zn, Mn)Te crystals and, as a result, increasing the charge

**3.** The key results have been obtained from the investigation of the electrical characteristics of the Ni/CdTe/Ni structures with a Schottky contact based on CdTe:Cl crystals with nearly intrinsic conductivity: (i) the *I-V* characteristics of the Schottky diode structure with low reverse leakage current at high-bias voltages can be quantitatively described in terms of the known physical models: the generation-recombination in the SCR, the processes under conditions of strong electric fields and currents limited by space charge; (ii) a rapid rise of the current at high direct voltages due to injection of minority carriers from the Schottky contact to the neutral part of the crystal and the modulation of its conductivity; (iii) at relatively high reverse bias, the processes in the "Ohmic" contact on the opposite side of

**4.** The investigation results, obtained for Schottky diode detectors based on CdTe and

**5.** The important conclusions about application of the CdTe layers with a Schottky contact in direct-conversion flat-panel X-ray image detectors have been established based on the research results of the electrical and detection characteristics of the fabricated diode structures: (i) the highly developed technology of the deposition of polycrystalline CdTe layers of large area with a surface-barrier structure in solar cells can be adapted to the fabrication of flat-panel X-ray image detectors. The presence of a barrier structure in the

comparable with those in a-Se photoconductors (*ρ* = 1012 Ω·сm 300 K); (ii) electric field strength in the space charge region of a barrier structure eliminates the problem of the collection of charge generated by X-ray photon absorption. In a CdTe diode structure, virtually full charge collection occurs independently of the applied voltage at the carrier lifetime *τ* > 10−7 s and uncompensated impurity concentration higher than 1014 cm−3; (iii) the detection efficiency of X-rays in the Al/p-CdTe diode structure at the maximum possible electron lifetime (a few microseconds) is 50–70 and 20–40% in the photon energy ranges

(∼10−6 s) and demonstrating significant differences in detection of the spectra from 241Am, 57Co, 133Ba and 137Cs isotopes, have shown a correlation between the low concentration of uncompensated donors and poor detection efficiency of the Cd0.9Zn0.1Te detectors with quite acceptable resistivity and carrier lifetime. The conducted measurements and calculations show that the concentration of uncompensated impurities in the range from 3 × 1010 to 3 × 1012 cm−3 is yet another obligatory condition for effective operation of X/γ-ray Schottky

Te crystals.

–1010 Ω·cm) and minority carrier lifetime

–106 Ω·сm) provides low-leakage (dark) currents

the crystal affect the reverse-biased Schottky contact.

Cd0.9Zn0.1Te crystals with high resistivity (∼109

diode detectors based on CdTe and Cd1−xZn<sup>x</sup>

relatively low resistivity CdTe (*ρ* = 104

carrier lifetime.

44 New Trends in Nuclear Science

This research was supported by the Collaborative Project COCAE (Grant SEC-218000) of the European Community's Seventh Framework Programme and by the Collaborative Project SENERA (Grant SfP-984705) of the NATO Science for Peace and Security Programme. The authors express gratitude to all colleagues indicated as co-authors in Refs. [5-11, 15, 20, 21, 23-28] for their contribution in carrying out the investigations.
